Concrete is the most widely used building material in the world, with untold amounts being produced yearly. It has always been regarded as a strong, solid, impenetrable, almost indestructible material yet it can make cracks that are vulnerable to penetration by water. As the result, structures of great economic and esthetic value are forever at risk. The cost of fixing this problem is staggering. Estimates are that fixing concrete highway bridges in the United States alone would amount to $4 billion a year. In addition to the expense, making cement is not environmentally friendly. The process generates about 7% of all the CO2 that is released into the atmosphere due to human action. (Incidentally, cement is the powder used to make concrete. What we walk on is concrete, not cement.)